This Application is related to automatic adjustment of collimating lens of a raster scanning system and more particularly to a raster scanning system which utilizes a collimating lens mounted on a dome shaped ceramic piezo-electric servo. The piezo electric servo automatically adjusts the distance between the laser diode and the collimating lens of the printing system to assure the light beam exiting the collimating lens is collimated.
Referring to FIG. 1, there is shown a tangential (fast-scan) view of the raster output scanner 10 of a printing system. The raster scanning system 10 utilizes a laser light source 12, a collimating lens 14, pre-polygon optics 16, a multi-faceted rotating polygon mirror 18 as the scanning element, post polygon optics 20 and a photosensitive medium 22.
The laser light source 12 sends a light beam 24 to the rotating polygon mirror 18 through the collimating lens 14 and the pre-polygon optics 16. The collimating lens 14 collimates the light beam 24 and the pre-polygon optics 16 focuses the light beam 24 in the sagittal or cross-scan plane onto the rotating polygon mirror 18. The facets 26 of the rotating polygon mirror 18 reflect the light beam 24 and also cause the reflected light beam 24 to revolve about an axis near the reflection point of the facet 26. The reflected light beam 24 can be utilized through the post polygon optics 20 to scan a document at the input end of an imaging system as a raster input scanner or can be used to impinge upon a photographic film or a photosensitive medium 22, such as a xerographic drum (photoreceptor), at the output of the imaging system.
The distance d between the laser light source 12 and the collimating lens 14 is critical to the quality of print. If for any reason such as thermal expansion of the fixtures, the distance between the laser light source 12 and the collimating lens 14 changes, the light beam existing the collimating lens 14 will not be collimated. This in turn causes the light beam 24 striking the photoreceptor 22 to be out of focus which would degrade the quality of the latent image.
Referring to FIG. 2, a traditional approach to resolve this problem is to mount the laser light source 30 on a fixture 32 and mount the collimating lens 34 on a second fixture 36. The two fixtures 32 and 36 are attached to each other via a fixture 37 which is attached to the housing 38 of the raster scanning system. The material and the length of fixture 32 are different than those of fixture 36. The difference in length and the material of the two fixtures 32 and 36 are selected to keep the distance d.sub.1 fixed. The combination of the expansion coefficients of the two materials with the difference in lengths will cause the two fixtures 32 and 36 to expand at the same rate.
However, this approach does not provide an accurate adjustment. Due to the impurities within the metals and the fabrication method, metals usually do not have a precise coefficient of expansion. Typically for each metal, the coefficient of expansion has a range where the user usually uses the average. Therefore, the inconsistencies in the coefficients of expansion of the two materials may cause significant errors in the above approach.
It is an object of this invention to provide a servo which is capable of automatically adjusting the distance between a light source and a collimating lens of a printing system when the light exiting the collimating lens looses its collimation.